Hard disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, the data is magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs. A read channel and interface circuit are provided to recover previously stored data from the discs to the host computer.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically used to control the position of the heads relative to tracks on the discs. The tracks are defined from servo information that is written to the surfaces of the discs during manufacturing. The servo system of a disc drive thus utilizes the servo information in the performance of two primary operations: seeking and track following.
Seeking entails the movement of a selected head from an initial track to a destination track. For seeks of a sufficient length, a velocity-control approach is typically employed wherein the velocity of the head is repetitively determined and compared to a velocity profile which defines an optimum velocity trajectory for the head as it moves to the target track. The amount of current applied to an actuator coil varies in proportion to the velocity error, the actuator coil being part of a voice coil motor used to control the position of the head.
Track following entails the continued positioning of a selected head over a corresponding, selected track. A position-control approach is typically employed wherein the relative position of the head with respect to the center of the track is determined and compared to a desired position for the head. The resulting position error is used to control the amount of current that is applied to the actuator coil in order to maintain the head at the desired position relative to the track.
As will be recognized, modern disc drives typically employ an embedded servo scheme wherein the servo information is angularly spaced and interspersed among user data fields (or "sectors") on the surfaces of the discs. However, the sampling rate of the servo information is typically insufficient to provide the gain necessary to maintain the heads within predetermined off track boundaries. Accordingly, a multi-rate observer is deployed to provide estimates of head position, velocity and bias at times when the heads are disposed over the user data fields. Thus, the servo system utilizes position information obtained from the discs to provide the observer with the input required to give estimates for controlling the movement of the heads during seeking and track following. Such observers (or "estimators") are well known in the art and are discussed, for example, in U.S. Pat. No. 5,585,976 issued Dec. 17, 1996 to Pham, assigned to the assignee of the present invention.
A continuing trend in the disc drive industry is to provide disc drives with ever increasing data storage and transfer rate capacities. Some disc drives of the current generation have track densities greater than about 3000 tracks per centimeter (about 8000 tracks per inch). As track densities continue to increase, it becomes increasingly important to provide servo systems that are capable of accurately positioning the heads during seeks and track following modes of operation. Various gains of the servo system are typically selected to achieve a certain control response for the servo system; however, changes in environmental conditions and other factors tend to affect the characteristics of the servo system during operation.
While efforts in the prior art to adapt the operational characteristics of disc drive servo systems in view of changing environmental conditions have been successful, there is a continual need for improvements whereby servo system performance can be enhanced in view of further advancements in the art, such as continued increases in disc drive track densities.